This invention relates generally to soil moisture measuring instruments commonly known as tensiometers, and, more particularly, to tensiometers which are intended to be used as portable instruments rather than installed permanently in an operative position inserted in the soil.
As is well known, if the moisture content of soil can be accurately monitored, irrigation can be controlled to produce a desired rate of plant growth. Instruments of the tensiometer type have been used for a number of years for this purpose. Basically, a conventional tensiometer comprises a sealed tube defining a chamber which is normally completely filled with water, a hollow porous tip on one end of the tube, and a vacuum gauge connected to the water chamber. The porous tip is inserted in the soil and establishes liquid contact between the water in the tube and films of moisture in the soil surrounding the tip.
Relatively dry soil tends to pull water from the tube through the porous tip, but, since the tube is sealed, only a minute amount of water is actually withdrawn, and the water in the tube is "stretched" by the pulling effect of the dry soil, thus creating a measurable subatmospheric pressure in the tube. Higher moisture contents in the soil produce correspondingly less vacuum in the tube, and completely saturated soil registers substantially zero vacuum.
Since it may take a substantial time for the water in the tensiometer tube to reach a stable equilibrium condition with respect to the moisture in the soil, many tensiometers are utilized as permanently installed instruments, so that immediate readings of the soil moisture content can be obtained. Clearly, however, there are significant economic disadvantages in having to purchase and maintain a relatively large number of tensiometer instruments. There is therefore also a need for portable tensiometers, which an be transported between selected measuring sites, and inserted in the soil to obtain the desired soil moisture measurements. Portable tensiometers can include a null-adjustment device which allows the equilibrium condition to be reached more rapidly than would otherwise be possible. Essentially, the null-adjustment device includes a chamber within the enclosed tensiometer water system, the volume of which is adjustable by a movable piston.
Even with the inclusion of such a null-adjustment device, however, portable tensiometers available prior to this invention have suffered one serious shortcoming. A portable tensiometer is usually carried between measuring sites in the cab of a truck or other vehicle, and may at these times be subjected to relatively high temperatures. The soil, on the other hand, will be relatively cool, and the instrument will therefore be subjected to a sudden lowering in temperature on insertion in the soil. Since water has a thermal coefficient of expansion which is typically many times greater than that of the metal tube usually used to enclose the water, this sudden lowering of temperature will result in a substantial contraction of the water within the tube. This, in turn, results in a substantial error in the vacuum measurement. Similar errors can result from largely unpredictable temperature variations, caused by moving clouds, for example, while a measurement is being taken. Since vacuum gauges used in portable tensiometers typically require a flow of only approximately 0.1 milliliter of water to register a full-scale change in pressure from atmospheric pressure (0 centibars) to a vacuum of 100 centibars, it will be appreciated that these temperature variations can result in substantial errors in the readings obtained.
Accordingly, there is a real need in the field of soil moisture measurement for a portable instrument which eliminates or minimizes errors resulting from changes in temperature to which the instrument is subjected. The present invention fulfills this need.